U.S. Pat. No. 3,859,529 (Re. No. 31,847) to G.W. Luckey discloses the uses of stimulable phosphors as a recording medium in radiography. The recording medium is first exposed to x-rays of wavelengh .lambda..sub.1 to form a latent image in the phosphor. The incoming flux of x-rays produces a number of excited electrons and holes in the phosphor, some of which are trapped in long-lived (storage) states within the phosphor. At a later time the phosphor may be destructively scanned by stimulating radiation of wavelength .lambda..sub.2 to produce a luminescent emission of wavelength .lambda..sub.3, which is proportional to the original x-ray exposure. The term "destructively" is used herein to denote that the phosphor is discharged by the stimulating radiation, and that only a finite amount of stimulated radiation is emitted by the phosphor, regardless of the quantity of stimulating radiation applied. The terms "storage phosphor(s)" and "phosphor(s)" as used herein, refer to phosphors that, upon stimulation, destructively release emitted radiation.
The above described system uses conventional x-ray exposure equipment. However, in place of the screen and film of conventional radiography a recording medium in the form of photo-stimulable luminescent storage phosphor is used to detect and record the image. After exposure the recording medium is scanned, in a raster pattern by a laser beam deflected by an oscillating or rotating scanning mirror, and the luminescent emission at wavelength .lambda..sub.3 is collected and detected by a photodetector such as a photomultiplier tube and converted to digital information which is transmitted to a computer which, in turn, processes the image. U.S. Pat. No. 4,778,995 to R.W. Kulpinski et. al. discloses in schematic form the basic method of scanning, in which the laser is fixed, the laser beam is deflected in the fast or line scan direction (in this case by a rotating polygon mirror) and the recording medium advanced in the slow or page scan direction by a suitable sheet drive mechanism.
Optically turbid (non transparent) phosphor storage mediums are used in commercial systems currently available. However, there are certain advantages that an optically transparent phosphor has over a turbid phosphor. Since the MTF (Modulation Transfer Function; a measure of the ability of the system to record details) of the transparent phosphor imaging system is limited mainly by the effective size of the scanning beam of stimulating radiation, which may be adjusted to a desired size, the MTF may be made much higher than in a comparable turbid phosphor system. In addition, the x-ray absorption of the sheet may be increased by making it thicker, without increasing the effective size of the scanning beam. In this way the signal-to-noise ratio of the x-ray detector may be improved. In the conventional turbid storage phosphor sheets, the thickness is limited by the spreading of the scanning beam in the turbid phosphor. Optically transparent storage phosphors are disclosed in U.S. Pat. No. 4,733,090 to C.D. DeBoer et. al.
In the practice of scanning a transparent storage phosphor, however, a new problem may arise as a result of the lack of stimulating radiation scattering in the phosphor. That is, if the index of refraction n.sub.2 (at wavelength .lambda..sub.2) of the medium following the transparent phosphor in the optical path is not exactly matched to the index n.sub.1 of the phosphor (at .lambda..sub.2), then some fraction of the stimulating beam 3 will be reflected back into the storage phosphor 5 at the n.sub.1 -n.sub.2 boundary, as indicated by 7 in FIG. 1. Beam 3 is generated by a laser (not shown), deflected by a conventional rotating mirror 8 and passes through a conventional f0 lens 9, which converts the constant angular velocity of beam 3 into a spot at the image plane that moves at constant linear velocity. Further, if the stimulating beam 3 is not directed perpendicularly to the storage medium but rather is incident at some finite angle to the normal, as is typical in raster scanning systems such as illustrated in U.S. Pat. No. 4,778,995 to R.W. Kulpinski et. al., then the reflected portion of the beam will be directed away from the area of the incident beam, as indicated in FIG. 1. The unwanted reflected light may continue on this path for some distance within the storage phosphor 5, giving rise to undesirable effects similar to those of "flare light" in many other optical systems. These effects include a degradation in image contrast, " shadow" artifacts, and a loss in output signal-to-noise ratio, particularly when scanning a low x-ray exposure region surrounded by regions of higher exposure. Further, since the storage phosphor is scanned destructively, the undesirable effects of flare light are actually enhanced compared to the case of non-destructive scanning, when the scanning is carried out at high laser power.
The object of the present invention is to solve the above described problem.